Design trends are increasingly at the fore in modern devices. As well as the visual impression made by a device, much importance is attached to constantly improving the ease of handling of the device concerned. At the same time, the miniaturization of portable electronic devices with inputting facilities is leading to devices with ever smaller surface areas and volumes, so advances in technological development, especially in mobile telephones, are leading to ever smaller devices overall. The design space available for keypads among other things is consequently becoming smaller and smaller, which can have a very detrimental effect on the user-friendliness of such devices.
A minimum volume of a keypad is determined by the following criteria: key surface area, key spacing and the projection of the keys relative to the housing, which projection influences the key height. The volume of keypads will be smallest where a respective key cap remains as flat as possible. The area of keypads will then be smallest where the key spacing approaches zero, i.e. the keys will no longer be separated from one another by housing support frames and will thus be individually held in a form-locking manner in the housing, as is known in PC keyboards.
Without restricting the scope of the invention to this application field, the embodiments disclosed below will be described in the context of mobile telephones and the input devices thereof, due to their small structural size, the heavy mechanical loading on them caused by dialing operations and especially by the sending of short written messages in the form of SMS messages, as well as to their being produced under very great cost pressures.
According to the prior art, input devices with keys are known, wherein the externally accessible caps of a user interface are each fitted on at least one side with a flange. The cap rests with at least one flange arranged against corresponding housing support frames in order to prevent it from falling or springing out. Without housing support frames, which are generally present between all the keys, such securing is possible only in an area of contact between keypad and housing or keypad and upper housing shell. In particular, the keys with the dial numbers 5 and 8 will thus in conventional mobile telephone keypads no longer be adequately secured to prevent the entire keypad, held in the housing without any support frames, from sliding out or being torn out.
In a known input device with individual keys not having support frames, the keys are fastened onto a flexible plastic carrier. The flexible plastic carrier must for its part be firstly mechanically very stable in order to secure the input device in the housing and/or to act as a backing system and secondly also highly flexible in order to ensure operability of the individual keys. These requirements are so conflicting that a workable compromise can only be found to a highly restricted degree. With housing dimensions continuing to shrink, long-term functional reliability is no longer guaranteed.
From EP 1 156 643 A2, a keypad for a mobile telephone is known, in which the keypad caps, not separated from one another by housing support frames, are fixed on a flexible, film-type carrier. The film carrier is fashioned for example as a silicon carrier. In one embodiment, the film carrier itself is bonded to a printed circuit board which for its part is held in an overlapping area in a form-locking manner by a surrounding housing by virtue of the fact that the film carrier has a circumferential flange. Accordingly, the result is a generally frame-like area of overlap between the flange and for example an upper housing shell in which the keypad is enclosed via combinations of hooks and eyes and/or pins and holes, held in a form-locking or force-locking manner and thereby fixed.
As a result of the miniaturization of devices overall, however, an overlapping area of contact between a respective keypad and a housing is becoming smaller and smaller, so as to ensure, without any further shrinkage in key size, a minimum degree of convenience of use and input reliability. Mechanical securing of a keypad or of a keypad module relative to a housing in the known form of interlocking by means of combinations of recesses or undercuts with corresponding support frames, hooks and/or studs is then no longer adequate. Simple and, in production engineering terms, readily implementable bonding of a keypad or keypad module to an upper shell for lasting protection of the keys to prevent the keys or the keypad carrier together with all the keys from falling or springing out has also proven inadequate.
As a result, a large amount of design space is needed in an area around the input device or the keypad of a mobile telephone which could better be used to increase user-friendliness by relatively increasing the size of the individual keys. The only alternative is to change the design to comply with currently technically implementable forms, for example to enlarge the housing solely in order to fix the keypad securely. In this way, however, the overall device would have a housing that was larger than was actually technically necessary. This approach to a solution would, however, stand in the way of miniaturization.